1. Field
The present disclosure generally relates to structural joints, especially bonded joints in composite structures, and deals more particularly with a device for controlling stress in the joint at low temperatures, and with a method of making the device.
2. Background
Certain structures may be specifically designed for use in cryogenic environments. For example, spacecraft and launch vehicles may employ tanks to store solid propellants or liquid fuels such as liquid hydrogen and/or liquid oxygen at cryogenic temperatures, typically below −238 degrees Fahrenheit. In order to reduce weight of the spacecraft or launch vehicle, the tank may be formed of several composite parts that are attached to each other by one or more joints. In one such application, the composite tank includes a cylindrical outer wall having a skirt joined to dome-shaped ends by bonded lap joints that are Y-shaped in cross section.
In composite cryogenic tanks of larger diameters, e.g. diameters of greater than approximately than 14 to 16 feet, the line loads in the bonded joints may result in unacceptably high levels of peak stresses in the joint, presenting the possibility of a joint leak. In order to increase the strength of the Y-joint, it may be possible to place a softening strip in a notch area of the joint. The softening strip may reduce shear stress peaking that occurs in the bond line by enabling a smoother load transmission between structures. This could be accomplished at ambient temperatures using a softening strip formed of a material such as rubber, however, at cryogenic temperatures, rubber and many other conventional materials may not remain soft, but instead harden. Upon hardening at cryogenic temperatures, the softening strip may no longer be capable of controlling shear stress in the joint.
Known materials that may be used as softening strips present several other problems in cryogenic environments. For example, such materials typically have a coefficient of thermal expansion (CTE) that may be incompatible with other composite components of the joint which have a relatively low CTE. This mis-match of CTE's may create undesirable thermally-induced stresses in the joint at cryogenic temperatures. Adding to the challenge of designing an effective softening strip is the fact that the softening strip must remain stable at the temperatures at which the other composite resin components of the tank are thermally cured. Also, it is desirable that that softening strip be able to be machined into a shape that is specific to the particular joint application.
Accordingly, there is a need for a softening strip device that is effective in controlling stresses in joints at cryogenic temperatures. There is also a need for a method of making a softening strip of the type mentioned above that is relatively simple, effective and well-suited for use in production environments.